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PostPosted: Sun Apr 22, 2018 5:46 pm 
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Hi people - I hope you can help me solve a frustrating issue.

I have an indirect, vented, S-plan central heating system.

If I run the boiler with the Hot Water on and the central heating off then the boiler overheats within about 3 mins.
If I run the boiler with the central heating on and the hot water off, then the radiators get nice and hot (pump is good).
If I run the boiler with the central heating on and the hot water on then the radiators still get nice and hot, but the hot water takes forever to get hot - it does get there in end, after maybe 4 hours - the boiler does not overheat.

I notice that the hot water flow pipe where it goes into the hot water cylinder is scorching hot (so the zone valve is opening OK), but the return pipe takes forever to heat up, I am not sure that when it does heat up its because of any water flowing through it, I think its just radiant heat from other pipes.

Thinking it might be a blocked coil or a airlock in the coil I have done the following:

- connected hose to radiator drain valve
- closed gate valve on cold feed from F and E tank so no water can flow up to F and E tank through cold feed pipe
- closed CH pump valves (probably not necessary)
- left the heating turned off to ensure both zone valves are closed
- blasted mains water up though radiator

By my reckoning this should have forced water through backwards through the return pipe of the coil and then up and out of the vent pipe over the F and E tank - and that is exactly where the water ended up coming out, but I think I'm missing something as the issue persists, the water that came out of the vent pipe was nice and clear and did not seem to be obstructed.

I'd be grateful for any advice.


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PostPosted: Sun Apr 22, 2018 11:35 pm 
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May be worth a try.
Can you see any air vents on any pipe work near the H/W cylinder.
If you can try bleeding any air out of them.


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PostPosted: Mon Apr 23, 2018 7:46 am 
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Thanks for the suggestion, unfortunately there are no air vents near the pipework to the cylinder.


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PostPosted: Mon Apr 23, 2018 8:05 am 
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Obviously you have a restriction in the HW circuit somewhere, my first suspicion would be the HW zone valve not opening properly.

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PostPosted: Mon Apr 23, 2018 8:08 am 
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Hi Dave thanks for the reply, so where the flow pipe enters the cylinder it is very hot, wouldn’t that indicate that the zone valve is opening Ok? Thanks!


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PostPosted: Mon Apr 23, 2018 5:50 pm 
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Does your cold feed from the expansion tank join in to the return of hot water cylinder?

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PostPosted: Mon Apr 23, 2018 6:47 pm 
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Yes it does.


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PostPosted: Mon Apr 23, 2018 8:39 pm 
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Then I would think the blockage is in that tee where the cold feed joins.

I would drain down then cut the cold feed a couple of inches above the tee and inspect it.

You may even have to cut the whole tee out.

If you have a magnet, put it on the tee and the pipe work near it. If it sticks to the tee you have found the blockage. Unfortunately if it doesn’t stick you probably still have a blockage from your description.

The pipe from the hw zone valve will get hot as it only needs a very slight flow to heat this up, whilst restricting the flow enough to stop the cylinder heating up.

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PostPosted: Mon Apr 23, 2018 9:39 pm 
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Thanks so much for the info Dave!

So, I have read before that the tee where the cold feed meets the return pipe can be troublesome and I did actually run a magnet around the area hoping it would stick, but I didn't have any luck - do you need a super strong magnet or anything or will a normal fridge magnet do the job?

Also, with blockages in that tee, do they tend to occur in the vertical bit from the cold feed or in the horizontal bit of the return pipe from the cylinder coil?

Lastly, with the zone valve.. I can move the switch underneath it over to the manual open mode, if it was blocked or obstructed would i be able to physically do that?

Really appreciate all your help - thanks mate.
Dave


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PostPosted: Mon Apr 23, 2018 10:05 pm 
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Obviously I can’t say 100% it is the tee, but it would be the first place now that I would check. The magnet trick doesn’t always work.

Usually if the system doesn’t fill then the blockage is in the cold feed part of the tee, but in your case I am guessing it will be somewhere in the return part of the tee.

Unfortunately the only way to check for certain is to drain and cut the pipes.

It could possibly be a waste of time, but it needs eliminating as the problem.

I personally have never had a zone valve block bad enough to not be able to move the lever manually.

Only you can now decide whether cutting and checking the return at the tee is worth it, but personally I would.

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PostPosted: Mon Apr 23, 2018 10:11 pm 
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Many thanks Dave. I’ve wasted a lot of time already so I don’t mind potentially wasting a bit more, like you say I can at least rule it out if then! Will let you know how i get on.

Thanks again!


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PostPosted: Sun Apr 29, 2018 8:09 pm 
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Hi there
I am finding it hard to post a problem with my boiler, how do I post a problem?


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